The present invention generally relates to heterojunction bipolar transistors (HBTs) for high power applications, and more particularly, to a heterojunction bipolar transistor having an emitter structure capable of reducing the current crowding effect and preventing thermal instabilities, wherein a negative differential resistance (NDR) element is added to the layer structure of the conventional emitter. The NDR element is designed to limit the tunneling current to the maximal emitter current density required for safe transistor operation.
The effect of current crowding is a major obstacle in the operation of conventional bipolar power transistors. Due to the relatively large sheet resistance of the transistor base layer, the lateral conduction of the base current causes an ohmic voltage drop which reduces the effective forward bias applied to regions of the emitter junction away from the base contact, as described in the paper by J. R. Hauser, xe2x80x9cThe Effects of Distributed Base Potential on Emitter Current Injection Density and Effective Base Resistance for Stripe Transistor Geometriesxe2x80x9d, IEEE Transactions on Electron Devices, ED 11, pp. 238-242, (1964). As the base current is increased, the lateral voltage drop becomes larger and the non-even distribution of the emitter current is increased.
The high density of the emitter and collector currents near the base contact causes significant localized heating. The elevated temperature causes an increase in the base collector junction leakage current, reduces the base emitter turn-on voltage, as described in the paper by R. H. Winkler, xe2x80x9cThermal Properties of High Power Transistorsxe2x80x9d, IEEE Transactions on Electron Devices, ED14, pp. 260-263, (1967) and modifies the transistor gain. If the transistor current gain changes are small, a positive feedback effect is generated. If the current of the device is not limited by the external circuit, this thermal runaway can destroy the transistor, as described in B. G. Streetman, xe2x80x9cSolid State Electronic Devicesxe2x80x9d, 3rd ed., Prentice Hall, Englewood Cliffs, N.J., pp. 271-272, (1990).
A common solution to the current crowding effect is the fabrication of a multiple finger transistor made of several thin fingers that are connected in parallel. Such devices are used for high power and microwave applications, as described in S. M. Zee, xe2x80x9cPhysics of Semiconductor Devicesxe2x80x9d, 2nd ed., Wiley, New York, pp. 165-166, (1981). Another advantage of this configuration is the improved high frequency operation due to the reduced effective base resistance.
However, when high power is applied to a multiple finger transistor the effects of thermal instability are exhibited, due to an uneven distribution of the current between the transistor fingers. These effects are described in the paper by H. F. Chau, W. Liu and E. A. Beam III, xe2x80x9cInP Based HBTs and Their Perspective for Microwave Applicationsxe2x80x9d, 7th International Conference on Indium Phosphide and Related Materials, pp. 640-643, (1995), and the paper by W. Liu, H. F. Chau and E. Beam III, xe2x80x9cThermal Properties and Thermal Instabilities of InP Based Heterojunction Bipolar Transistorsxe2x80x9d, IEEE Transactions on Electron Devices, 43, pp. 388-395, (1996). Also, the effect of the current gain collapse takes place, as described in the paper by W. Liu, S. Nelson, D. G. Hill and A. Khatibzadeh, xe2x80x9cCurrent Gain Collapse in Microwave Multifinger Heterojunction Bipolar Transistors Operated at Very High Power Densitiesxe2x80x9d, IEEE Transactions on Electron Devices, 40, pp. 1917-1927, (1993), and the paper by W. Liu and A. Khatibzadeh, xe2x80x9cThe Collapse of Current Gain in Microwave Multi-finger Heterojunction Bipolar Transistors: Its Substrate Temperature Dependence, Instability Criteria and Modelingxe2x80x9d, IEEE Transactions on Electron Devices, 41, pp. 1698-1707, (1994).
The thermal instability occurs since the base collector leakage current increases and the transistor turn-on voltage decreases with the rise in temperature. The hottest finger gradually dominates the device operation by drawing higher portion of the total current. The increased current flow causes further heating and a destructive regenerative process is generated.
This thermal instability can be stabilized at the cost of an increased emitter resistance, by adding a stabilizing ballast resistor to each of the emitter fingers, as mentioned in the above-referenced book by S. M Zee (pp. 169-170). Another approach is the incorporation of a high resistance n-type layer in the emitter, as described in the patent by W. Liu and D. G. Hill, xe2x80x9cMicrowave heterojunction bipolar transistors with emitters designed for high power applications and method for fabricating samexe2x80x9d European patent EP 0562272, (1993). It is important to note that high emitter resistance deteriorates the microwave performance of the device.
While it has been previously proposed to integrate a Resonant Tunnel Diode (RTD) in the emitter of bipolar transistors, for the purpose of multilevel logic design, its application in reducing the current crowding effect and preventing thermal instabilities has not been previously discussed. The known applications of RTD in a multilevel logic design are described in the paper by F. Capasso, S. Sen, A. Y. Cho and D. L. Sivco, xe2x80x9cMultiple Negative Transconductance and Differential Conductance in a Bipolar Transistor by Sequential Quenching of Resonant Tunnelingxe2x80x9d, Applied Physics Letters, 53, pp. 1056-1058, (1988), also in the paper by S. Sen, F. Capasso, A. Y. Cho and D. L. Sivco, xe2x80x9cMultiple-State Resonant-Tunneling Bipolar Transistor Operating at Room Temperature and its Applications as a Frequency Multiplierxe2x80x9d, IEEE Electron Device Letters, 9, pp. 533-535, (1988), and in the paper by F. Capasso, S. Sen, F. Beltram, L. M. Lunardi A. S. Vengurlekar, P. R. Smith, N. J. Shah, R. J. Malik and A. Y. Cho, xe2x80x9cQuantum Functional Devices: Resonant Tunneling Transistors, Circuits with Reduced Complexity, and Multiple-Valued Logicxe2x80x9d, IEEE Transactions on Electron Devices, 36, pp. 2065-2082, (1989).
Therefore, it would be desirable to provide a solution to the operational problems of high power bipolar transistors which are related to the current crowding effect and thermal instabilities.
Accordingly, it is a principal object of the present invention to overcome the problems associated with the high power operation of bipolar transistors, by proposing and demonstrating a modified emitter structure which includes a Negative Differential Resistance (NDR) element added to the emitter layers. The NDR element limits the emitter current density, reduces the current crowding effect in a large area device and prevents the thermal instabilities in multiple finger transistors. The modified emitter design demonstrates the reduction of the current crowding effect in a large area device and the self current limiting property of a small area device. Both properties can be used to enhance the high power performance of the transistor and prevent thermal instabilities in a multiple finger transistor configuration.
In accordance with a preferred embodiment of the invention, there is provided a bipolar transistor having improved high power performance by reduction of the current crowding effect, said transistor comprising:
a substrate;
a sub-collector layer;
a collector layer;
a base layer; and
an emitter layer structure comprising a negative differential resistance element.
In a preferred embodiment, a resonant tunnel diode (RTD) or an Esaki diode is used as the negative differential resistance element. The maximal density of the forward tunneling current in a resonant tunnel diode (RTD) or an Esaki diode is determined by the device design. When such diode is added to the emitter of a bipolar transistor a composite device is formed where the maximal emitter current density is self-limited.
In a large area transistor, the limit on the emitter current density reduces the current crowding effect. This results from the fact that the peak concentration of the emitter current can not rise beyond the maximal tunneling current density of the diode.
In a small area device the total current is self-limited. Since a multiple finger transistor consists of a large number of small area devices connected in parallel and since each finger self-limits its total emitter current, the thermal instability is prevented. This is accomplished without adding ohmic resistance to all of the fingers (also called ballast resistors), thus reducing the total emitter resistance of the multiple finger transistor.
Since the current crowding effect is reduced, the size of each finger in a multiple finger transistor can be increased. Thus, the total number of fingers can be reduced and the transistor fabrication can be simplified.
Without limiting the scope of the invention, two types of NDR structures are discussed. In the first, an RTD is added to the emitter. The second structure consists of a forward biased Esaki diode, and a second, reversed biased Esaki diode, which is required to restore the conduction band current flow in a NPN transistor and valence band current flow in a PNP transistor.
In addition, a high resistance p-type layer can be grown between the two Esaki diodes used in a NPN transistor current limiter design. This layer can provide the emitter ballast resistance, forming a multiple finger transistor stabilizing mechanism which combines both standard ballast resistor and the disclosed NDR technique. Since the hole mobility is much smaller than the mobility of the electrons, the Ohmic resistance of p-type layers is larger than the resistance of an n-type layer with similar thickness and doping concentration. It is usually impractical to use standard n-type layers for providing the ballast resistance, due to the large thickness that is required. P-type layers, on the other hand, have larger resistance and the required layers are much thinner. P-type layers can be added to a standard emitter of a PNP transistor. In a NPN transistor, the use of an Esaki diode is required. The Esaki diode converts the conduction band current flow to the valence band current flow, making the integration of p-type layers possible.
Additional features and advantages of the invention will become apparent from the following drawings and the description.